For my last TV repair, I had to determine which LED was bad in a strip. I had found a youtube video where the poster had made a jig with two 1.5V batteries and sewing needles to pierce the soldermask but it doesn’t have to be that complicated. You just need a DMM and a razor blade/sharp knife.
On a set of LED strips I salvaged, there are actually test points that are just openings in the solder mask. By setting your DMM to diode test mode (or resistance at 2000 range), and touching the test points, the diode will glow if working. Switch the polarities if it doesn’t seem to be working. Also, don’t confuse the other dots for test points. They are fiducials used for alignment by the pick and place machine when the strip was constructed and are not connected to anything.
If you don’t have test points, you can scrape a bit of the soldermask away on one side of the LED. You just need to expose the copper underneath. Since the LED’s are in series, you don’t need two scrapes for every LED, just one between each LED.
With a series circuit, one LED going out will take out the whole strip. I removed an LED from a spare strip from the same size TV I had and swapped it. Replacement is a bit tricky since it is hard to align the LED with the narrow spacing on the pad so it is easy to short. The LED’s also have different voltage and color temperature specifications so you need to find one similar. The same brightness tested with the DMM will indicate it is a close match.
Strips are typically glued to the light box. and need to be remove with a scraper. To remove the reflector sheet hiding the strips, flip the box over and use pliers to squeeze the end of the retention clips poking through that are holding the sheet down.
Since I am seeing a lot more LED failures, I decided to buy a LED backlight tester from Banggood (was cheaper than ebay). Make sure you get one that is 120VAC for North America and 200VDC max is usually good enough for TV backlight testing.